The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
Not applicable
(1) Field of the Invention
This invention generally relates to an electrolyte additive for aluminum anode activation.
More particularly, the invention relates to an electrolyte additive for aluminum anode activation in which the additive is sodium gallium oxide, the additive preventing or reducing the formation of an oxide coating on a surface of a metal.
(2) Description of the Prior Art
In the current art of aluminum based semi-fuel cells (Al-SFC), elemental aluminum (or one of its alloys) along with hydrogen peroxide, is consumed to produce energy. Among the more promising semi-fuel cells currently being considered as electrochemical sources is the aluminum/hydrogen peroxide cell. The type of aluminum used in the semi-fuel cell is dictated by the specific requirements of the application. Applications requiring high discharge rates (current densities above 1000 mA/cm2), typically utilize aluminum-based alloys such as XA5-P and DF50V, while for low rate (current densities from 5-50 mA/cm2) applications, EB50V is the aluminum alloy of choice.
All three proprietary alloys are formulated by ALCAN International; however, consideration of these alloys for further use is jeopardized by the following facts: (a) EB50V, XA5-P, and DF50V are proprietary alloys manufactured by a sole source, and (b) the present state of the economy (supply and demand) has forced the cost of these unique alloys to prohibitive and costly levels beyond acceptable acquisition levels. For these reasons, pure aluminum has been investigated as a replacement for the costly and difficult to acquire proprietary alloys.
The use of pure aluminum metal (especially in low rate semi-fuel cell systems) is hindered by the fact that aluminum readily oxidizes in a caustic electrolyte, thereby forming a passive surface layer that causes its chemical reactivity to greatly diminish, and adversely affecting the power output and efficiency of the semi-fuel cell. It was found by the inventors that an addition of gallium ions to the electrolyte solution prevents aluminum oxide formations, and hence eliminates the problem of passivity.
The use of electrolyte additives to modify the chemical reactively of aluminum metal and aluminum-based alloys used as anodes in semi-fuel cells has been previously investigated for high rate applications in each of the following publications:
Enhanced Electrochemical Performance in the Development of the Aluminum/Hydrogen Peroxide Semi-Fuel Cell by E. G. Dow et. al., Journal of Power Sources 65 (1997) pp. 207-212.
Aluminum-Hydrogen Peroxide Battery Development: Part IIxe2x80x94Anode Polarization of Pure Aluminum Via Electrolyte Additives, Seebach et. al., Technical Memorandum of NAVAL UNDERWAEA WARFARE CENTER DIVISION NEWPORT, RHODE ISLAND, 15 Jun. 1992.
Electrochemical Characterization of aluminym alloy EB50V: The Effect of Sodium Hydroxide Concentration, Aluminate Concentration, Stannate Concentration, and Temperature, Medeiros et al., 18 Jan. 1993, Technical Memorandum of NAVAL UNDERWAEA WARFARE CENTER DIVISION NEWPORT, RHODE ISLAND.
For the most part, these studies utilized half-cell reaction experiments to obtain polarization data (i.e., current-voltage curves) that was correlated to changes in aluminum activity as a function of electrolyte additive Several electrolyte additives in the form of metal oxides were tested and it was found that gallium oxide yielded the best anodic voltage, xe2x88x921.3 volts versus Af/AgCl at 400 MA/cm2. However, since gallium oxide (Ga2O3) has limited solubility in caustic (seawater/sodium hydroxide) electrolytes typically used in aluminum based semi-fuel cells, it is difficult t,o quantify and/or control the effect that the gallium ion has on the electrochemical performance of aluminum, i.e. to determine the optimum gallium concentration required to prevent aluminum passivity. Accordingly, a need still exists in the art for a suitable additive having the desired properties.
The following patents, for example, discuss the prevention of corrosion by producing a protective oxide coating on the surface of a metal such as aluminum. However, these patents do not teach the prevention of formation of such a surface in the first place as does the present invention.
U.S. Pat. No. 3,347,155 to Weber;
U.S. Pat. No. 3,887,399 to Gunn; and
U.S. Pat. No. 6,030,517 to Lincot et al.
Specifically, Weber discloses a process of improving the corrosion resistance of aluminum articles that includes removing the impurities from the article surface, then chemically or electrolytically forming an artificial aluminum oxide coating, treating the artificially oxide coated article to a dilute aqueous solution of an inorganic base such as NaOH or KOH, and thereafter treating the article to an alkaline silicate solution. Advantageously intermediate the above mentioned treatments, the article is treated to one or more of aqueous solutions of (1) organic compounds having cations of various iron group metals and anions of acetates, citrates, oxalates, tartrates, (2) organic compounds of various alkali and alkali earth metals having anions of acetates, citrates, oxalates, (3) ammonium hydroxide, (4) ammonium compounds having an anion of such acetates, citrates, carbonates, and (5) various mixtures of the above.
The patent to Gunn discloses a multi-chambered incinerator having high temperature electric heater elements at one or more flame ports. The incinerator has a main combustion chamber followed by one or more additional chambers connected by one or more flame ports. In the flame port that may have checkerboarded refractory or a high temperature, an electric heater grid system of elongated heater elements is installed. The electric heater elements are designed for rapid rise in temperature, for example in a period of 5to 15 minutes to provide flame port temperatures in the order of 1300xc2x0 F. more or less depending on operating conditions. The elongated electric heater elements, which can be arranged either vertically or horizontally or as a grid system, provide an extremely rapid rise high temperature heating element to facilitate the combustion of waste materials and gases and particulates and further serves as an impingement screen to provide for settling of incombustible particulates. The electric heater system can be used with or without checkerboard refractory in the flame ports and provides an improved and efficient means for incinerating industrial, commercial or agricultural waste material and minimizes air pollution.
The patent to Lincot et al. discloses a process for depositing a film of a metal oxide or that of a metal hydroxide on a substrate in an electrochemical cell, wherein
(i) the metal hydroxide is of formula M(OH)xAy, M representing at least one metallic species in an oxidation state i chosen from the elements in Groups II and III of the periodic Table, A being an anion whose number of charges n, 0 less than x I and x+ny=I,
(ii) the electrochemical cell comprises (a) an electrode, comprising the substrate, (b) a counter-electrode, (c) a reference electrode and (d) an electrolyte comprising a conducting solution comprising at least one salt of the metal M, the process comprising the steps of: dissolving oxygen in the electrolyte and imposing a cathode potential of less than the oxygen reduction potential and greater than the potential for deposition of the metal M in the electrolyte in question on the electrochemical cell.
It should be understood that the present invention would in fact enhance the functionality of the above patents as follows: In contrast to the aforementioned patents, the intent of the present invention is not to produce a protective oxide coating on the surface of a metal such as aluminum but instead to prevent or reduce one such oxide coating from forming. In a semi-fuel cell (SCF), aluminum reacts with an alkaline solution such as sodium hydroxide to form an unwanted aluminum oxide layer. Since this oxide layer inhibits the electrochemical reactivity of the aluminum resulting in lower semi-fuel cell efficiency, it must be eliminated or minimized. The invention described herein utilizes a specific ternary compound, sodium gallate (AnGaO2) that will dissolve in alkaline electrolytes and in the presence of aluminum metal will prevent an oxide coating from forming on the surface of the aluminum.
Therefore it is an object of this invention to provide an electrolyte additive for aluminum anode activation.
Another object of this invention is to provide an electrolyte additive for aluminum anode activation in which the additive prevents or reduces formation of an oxide coating on a surface of a metal.
Still another object of this invention is to provide an electrolyte additive for aluminum anode activation in which the additive is sodium gallium oxide.
In accordance with one aspect of this invention, there is provided an additive for an aluminum-based semi-fuel cell system includes a combination of components including gallium, oxygen, and a sodium component dissolvable an alkaline electrolyte solution such as seawater and sodium hydroxide. These components form sodium gallium oxide and prevent formation of an oxide layer on a surface of an aluminum anode in the alkaline electrolyte of the semi-fuel cell system.